“From today on we cannot continue with business as usual, we must fight!” said lawmaker Fernando Cheung, standing in solidarity with his disqualified colleagues.

“We cannot pretend this legislature is normal.”

Another MP encouraged the crowd to return the same candidates in byelections that will eventually be held, although is no clear timeline determining when the vacated seats will be filled.

Lily Ng, who came straight from work to the rally, was enraged that Law, the candidate she voted for, was now disqualified.

“Ordinary people don’t have a lot of power in Hong Kong, and now this most important right has been stolen from us,” she said. “What’s the point of elections if they are meaningless?”

Justice Thomas Au wrote in his judgment that Law’s comments before his oath showed his was insincere. He had prefaced his oath with a quote from Gandhi and a pledge to serve the Hong Kong people.

He added that Lau’s slow-motion reading of the oath – she paused for six seconds between each word, taking 10 minutes to read the 77-word declaration – would “lead a reasonable person to conclude that she did not intend to convey any meaning of the contents and pledges of the LegCo [legislative council] oath when reading it out.”

In barring “Long Hair” Leung – who held a yellow umbrella during his oath, a symbol of the city’s 2014 democracy protests – Au wrote: “The manner in which Mr Leung took the oath goes well outside an objective reasonable range of such requisite solemnity and sincerity.”

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Yiu added words into the middle of his first attempt to take the oath, rendering it invalid in the eyes of the law, Au ruled.

The disqualifications come after two popularly elected pro-independence legislators were ejected in November. Their oaths, in which they pledged allegiance to the “Hong Kong nation” and used an expletive to refer to China, were ruled invalid and they were barred from taking their seats.

The protest by Yau Wai-ching and Sixtus Leung prompted Beijing to rewrite the Basic Law, Hong Kong’s mini-constitution, in a rare and highly controversial move that sparked street protests.

The move was the most direct intervention in the city’s politics since Britain handed over Hong Kong to China in 1997, and dealt a major blow to a campaign led by the city’s younger generation for greater autonomy or outright independence.

The Chinese government declared those wishing to hold public office must “sincerely and solemnly” declare allegiance to China. Lawmakers are required to swear allegiance to the “the Hong Kong special administrative region of the People’s Republic of China”.

But for years it has been a tradition among the pro-democracy camp, which has never held a majority in parliament, to add small acts of defiance during the swearing-in ceremony.

Lau’s slow reading of the oath had previously been ruled invalid, but she was allowed to retake the oath and sworn in. She later wrote on Facebook that the slow-motion reading was meant to show the affirmation was “meaningless”.

Leung Kwok-hung holds an umbrella while taking his oath at the legislative council. Photograph: Bobby Yip/Reuters

Lawmakers who are disqualified would have to be replaced in a byelection, which is supposed to occur within 21 days after all appeals are exhausted. That could take months as the cases will eventually be heard by Hong Kong’s highest court.

“Future pro-democracy candidates will be scrupulous in all their messaging, and in their oath-taking, to avoid giving the authorities any excuse to disqualify,” Pepper added. “Candidates and voters will learn to do what they have already begun to do: read between the lines of the campaign materials and party manifestos and make choices accordingly.”

Mabel Au, director of Amnesty International Hong Kong, said the decision was the “latest damaging sign that expressing political opinions that challenge the status quo are no longer tolerated.”

Barring Yau and Sixtus Leung in November, the judge said the pair “manifestly refused … to solemnly, sincerely and truly bind themselves” to Hong Kong’s laws, citing statements the judge said showed they made “a wilful and deliberate attempt … to insult China”.